ext4: Fix comparison endianness problem in MMP initialization

As part of startup, the MMP initialization code does this:

mmp->mmp_seq = seq = cpu_to_le32(mmp_new_seq());

Next, mmp->mmp_seq is written out to disk, a delay happens, and then
the MMP block is read back in and the sequence value is tested:

if (seq != le32_to_cpu(mmp->mmp_seq)) {
	/* fail the mount */

On a LE system such as x86, the *le32* functions do nothing and this
works.  Unfortunately, on a BE system such as ppc64, this comparison
becomes:

if (cpu_to_le32(new_seq) != le32_to_cpu(cpu_to_le32(new_seq)) {
	/* fail the mount */

Except for a few palindromic sequence numbers, this test always causes
the mount to fail, which makes MMP filesystems generally unmountable
on ppc64.  The attached patch fixes this situation.

Signed-off-by: Darrick J. Wong <djwong@us.ibm.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>

1 files changed, 2 insertions, 1 deletions

diff --git a/fs/ext4/mmp.c b/fs/ext4/mmp.c
index 6b327423e622..7ea4ba4eff2a 100644
--- a/fs/ext4/mmp.c
+++ b/fs/ext4/mmp.c
@@ -296,7 +296,8 @@ skip:
         /*
          * write a new random sequence number.
          */
-        mmp->mmp_seq = seq = cpu_to_le32(mmp_new_seq());
+        seq = mmp_new_seq();
+        mmp->mmp_seq = cpu_to_le32(seq);
 
         retval = write_mmp_block(bh);
         if (retval)